The correct answer to this open question is the following.
The source that would help a historian best understand the effect of the Civil War on soldiers' families is an oral history dictated by a soldier's adolescent son.
Primary sources are very important when doing research. A primary source is the source of information provided by a person that lived in the time when the event occurred or directly witnessed the event or the incident. That is why an oral history dictated by a soldier's adolescent son is a source that would help a historian best understand the effect of the Civil War on soldiers' families. He was the witness of what happened during the Civil War.